Good Day! Can you provide more info on how you are creating and adding the Sub-assets object?
Are you using a Microflow to Add a sub assets object or the mendix's default New button from DataGrid?
Are you using a Microflow to Save/Commit the sub-asset object or the default Save button?
Also did you make sure that the association between the Asset and Sub-asset object is set during Create Activity of the Sub-asset.
EDIT your post with some snapshot of your current implementation for better solution. Thanks. FYI, the association table doesnt populate on its own unless you set the association in a microflow or when the object is getting created from a Dataview with the association source.